This invention is directed to a xylene isomerization process.
Para-xylene is a valuable chemical feedstock which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C8 aromatic fractions from these sources vary quite widely in composition but will usually contain 10 to 32 wt. % ethylbenzene (EB) with the balance, xylenes, being divided between approximately 50 wt. % meta and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization or selective adsorption (e.g., the Parex process).
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C8 aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed to useful products while simultaneously minimizing any conversion of xylenes to other compounds.
One known method for removing ethylbenzene from a C8 aromatic stream is by dealkylation in which the ethylbenzene is converted to benzene and ethylene, with the latter normally being hydrogenated to produce ethane. Another known method for removing ethylbenzene is by isomerization to produce additional xylenes, normally through the intermediate step of saturating the ethylbenzene to produce naphthenes. In the past, a single catalyst was used to effect both xylene isomerization and ethylbenzene conversion, but this necessarily involved compromising between the different catalytic requirements of the two reactions. More recently, processes have been developed which employ separate catalysts tailored specifically for the different catalytic functions.
For example, U.S. Pat. No. 4,899,011 describes a xylene isomerization process employing ethylbenzene dealkylation, in which a C8 aromatic feed, which has been depleted in its para-xylene content, is contacted with a two component catalyst system. The first catalyst component selectively converts the ethylbenzene by deethylation, while the second component selectively isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The first catalyst component comprises a Constraint Index 1-12 molecular sieve, such as ZSM-5, which has an ortho-xylene sorption time of greater than 50 minutes based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120xc2x0 C. and an ortho-xylene partial pressure of 4.5xc2x10.8 mm of mercury, whereas the second component comprises a Constraint Index 1-12 molecular sieve which has an ortho-xylene sorption time of less than 10 minutes under the same conditions. In one preferred embodiment, the first catalyst component is ZSM-5 having a crystal size of at least 1 micron and the second catalyst component is ZSM-5 having a crystal size of 0.02-0.05 micron. Each catalyst component also contains a hydrogenation component, preferably a platinum group metal.
An improvement over the process of U.S. Pat. No. 4,899,011 is described in U.S. Pat. No. 5,689,027 in which the first catalyst component in the two component system is pre-selectivated by coking, or more preferably by deposition of a surface coating of silica, to increase its ortho-xylene sorption time to greater than 1200 minutes under the same conditions as cited in the ""011 patent. Using such a system it is found that high ethylbenzene dealkylation rates can be achieved with significantly lower xylene losses than obtained with the process of the ""011 patent.
Although the first and second catalyst components of the systems described in U.S. Pat. Nos. 4,899,011 and 5,689,027 can be housed in separate reactors, these processes are usually practiced in a single reactor in which the different components form separate beds in, for example, a fixed, stacked bed reactor. In contrast, U.S. Pat. No. 5,705,726 describes a similar process in which the ethylbenzene dealkylation step is performed in a separate reactor from that used for the subsequent xylene isomerization step. In theory, such a two reactor system offers significant advantages over a stacked bed system in that it allows the operating conditions as well as the catalyst properties to be tailored for the different reactions involved. In this way, it should be possible to operate at high ethylbenzene conversion while the xylene isomerization step is conducted at the milder conditions necessary to minimize reduce xylene losses. In practice, however, two reactor systems have generally not been adopted at least in part because of the increased capital cost of installing a second reactor and associated equipment.
The present invention seeks to provide a process which allows xylene isomerization and ethylbenzene conversion to be conducted in separate reactors without significant increase in capital cost by utilizing space within an existing reactor to accommodate the xylene isomerization catalyst. In particular, the invention is based on the realization that the product from the xylene isomerization step is normally fed to a clay treater to effect removal of any trace olefins in the product and that recent advances in olefin removal catalysts have significantly reduced the amount of catalyst required in the clay treater. As a result the clay treater provides reactor space which is already available in a conventional aromatics plant and which is suitable for accommodating a xylene isomerization catalyst. In addition, since the clay treater is operated at mild conditions compared with those employed in conventional xylene isomerization processes, the xylene losses can be reduced to very low levels.
Accordingly, the invention resides in one aspect in a process for isomerizing xylenes in a feed containing xylenes, which process comprises the step of contacting the feed with an isomerization catalyst under conditions effective to isomerize xylenes in the feed, wherein said isomerization catalyst is contained in a reactor which contains a further catalyst effective under said conditions to remove olefins in said feed.
Preferably, said conditions are such as to maintain said feed at least partially in the liquid phase.
Preferably, said conditions include a temperature of about 250xc2x0 F. to about 500xc2x0 F. (about 120xc2x0 C. to about 260xc2x0 C.), a pressure of about 50 to about 1000 psig (445 to 7000 kPa) and WHSV of about 0.1 to about 100.
More preferably, said conditions include a temperature of about 320xc2x0 F. to about 450xc2x0 F. (about 160xc2x0 C. to about 232xc2x0 C.), a pressure of about 100 to about 500 psig (790 to 3550 kPa) and WHSV of about 1 to about 30.
Preferably, said isomerization catalyst comprises a porous crystalline material selected from ZSM-5, MCM-22, MCM-36, MCM-49 and MCM-56.
Preferably, said isomerization catalyst has an alpha value greater than 300.
Preferably, said further catalyst comprises a porous crystalline material having pores and/or surface pockets defined by a ring of ten or more tetrahedrally coordinated atoms.
Preferably, said further catalyst comprises MCM-22.
Alternatively, said further catalyst comprises clay.
In another aspect, the invention resides in a process for isomerizing a feed which contains ethylbenzene and xylene, which process comprises the steps of:
(a) contacting the feed with a first catalyst in a first reactor under ethylbenzene conversion conditions, wherein the first catalyst is effective under said ethylbenzene conversion conditions to convert ethylbenzene in said feed and produce an ethylbenzene-depleted product; and then
(b) contacting the ethylbenzene-depleted product with a second catalyst in a second reactor separate from the first reactor under conditions effective to isomerize xylenes in the feed, wherein said second reactor also contains a third catalyst effective under the conditions in said second reactor to remove olefins in said feed.
In yet another aspect, the invention resides in a process for isomerizing a feed which contains ethylbenzene and xylene, which process comprises the steps of:
(a) contacting the feed with a first catalyst in a first reactor under ethylbenzene conversion conditions, wherein the first catalyst is effective under said ethylbenzene conversion conditions to convert ethylbenzene in said feed and produce an ethylbenzene-depleted product; and then
(b) contacting the ethylbenzene-depleted product with a second catalyst in a second reactor separate from the first reactor under conditions effective to isomerize xylenes in the feed and also remove olefins in said feed, wherein the conditions in said second reactor include a temperature of about 250xc2x0 F. to about 500xc2x0 F. (about 120xc2x0 C. to about 260xc2x0 C.), a pressure of about 50 to about 1000 psig (445 to 7000 kPa) and WHSV of about 0.1 to about 100.
In one embodiment of the invention, said first catalyst converts ethylbenzene by dealkylation. In such a case, the first catalyst preferably comprises an intermediate pore molecular sieve, such as ZSM-5, and has an ortho-xylene sorption time of greater than 50 minutes, and more preferably greater than 1200 minutes, based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120xc2x0 C. and an ortho-xylene partial pressure of 4.5xc2x10.8 mm of mercury. The first catalyst would normally also contain a hydrogenation component, such as platinum, palladium and/or rhenium.
In an alternative embodiment of the invention, the first catalyst converts ethylbenzene by isomerization to xylenes. In such a case, the first catalyst is preferably selected from the group consisting of platinum on alumina, platinum-containing, potassium-exchanged zeolite L, platinum-containing mordenite, platinum-containing SAPO-11 and platinum-containing ETS-10.